Furnance enclosure for natural circulation generator

ABSTRACT

A natural circulation generator having a rectangular furnace enclosure. A plurality of parallel, upright, finned tubes welded together along their lengths make up the walls of the enclosure. Burners are provided in the front wall of the enclosure, in the lower portion thereof, providing a heat input to the finned tubes. In accordance with the present invention, the tubes have a first diameter in the lower portion of the furnace, and a second greater diameter in the upper portion of the furnace. The increased tube diameter in the upper portion of the furnace gives a higher circulation ratio in the generator, and in addition provides more heat absorption surface area which increases the throughput steam flow per hour-foot of furnace periphery. This enables construction of a natural circulation unit having increased capacity.

Stevens 51 May 16, 1972 FURNANCE ENCLOSURE FOR NATURAL CIRCULATIONGENERATOR inventor: William D. Stevens, North Caldwell, NJ.

Assignee: Foster Wheeler Corporation, Livingston,

Filed: Dec. 14, 1970 Appl. NO.: 97,700

US. Cl. ..l22/6 A, 122/235 C, 122/406, 165/147 Int. Cl ..F22d 7/00 Field01 Search 1 22/6 A, 235, 235 A, 333, 406 R, 122/235 C; 165/146, 147

References Cited UNlTED STATES PATENTS 3,060,908 10/1962 Brister et a1..l22/235 X Primary Examiner-Kenneth W. Sprague Attorney-John Maier,[11, Marvin A. Naigur and John E. Wilson [57] ABSTRACT A naturalcirculation generator having a rectangular furnace enclosure. Aplurality of parallel, upright, finned tubes welded together along theirlengths make up the walls of the enclosure. Burners are provided in thefront wall of the enclosure, in the lower portion thereof, providing aheat input to the finned tubes. In accordance with the presentinvention, the tubes have a first diameter in the lower portion of thefurnace, and a second greater diameter in the upper portion of thefurnace. The increased tube diameter in the upper portion of the furnacegives a higher circulation ratio in the generator, and in additionprovides more heat absorption surface area which increases thethroughput steam flow per hour-foot of furnace periphery. This enablesconstruction of a natural circulation unit having increased capacity.

11 Claims, 3 Drawing Figures FINISHING SUPER HEATER OUTLETS REHEATEROUTLET EHEATER INLET PKTENTEnnmsmrz 3.662.716

Fl HIN UPER HE ER TLETS REHEATER OUTLET l E I REHEATE R INLET WILL/AM D.STEVENS Rmmno H- THOM As ATTORNEY INVENTOR.

FURNANCE ENCLOSURE FOR NATURAL CIRCULATION GENERATOR The presentinvention relates to natural circulation generators, and particularly toan improved furnace arrangement which enables the construction ofnatural circulation generators of higher capacity.

Natural circulation generators heretofore have been limited in sizeprimarily because of the head available for circulation. The head ormotive force in a natural circulation unit is dependent upon thedifference in density between the flow in the downflow circuit and thatin the upflow circuitry, minus losses from friction, shock, turbulenceand other factors, which losses increase with increased capacity orsize, Also, relatively high pressures are employed today in naturalcirculation units, further causing a reduction in the head available forcirculation.

One measure of the circulation or head in a generator is the circulationratio, or ratio of weight rate of water fed to the steam generator tubesdivided by the weight rate of steam generated..When natural circulationis the sole motive force in the generator, variations in heat transferor heat absorption in wall tubes of the generator, caused either bydifferent heat intensities in the furnace periphery, or by the use ofdifferent lengths of tubes or circuits, require that the circulationratio be high enough for the entire generator so that the leastfavorable furnace wall circuit receives an adequate water supply. Thisrequired margin of safety imposes a further limitation on the size ofunit which can be built.

Still further, present day generators conventionally are topsupported.Limitations on the thickness of tube wall which can be used, andmaterials available, taken together with the need for adequate support,have heretofore limited the size of unit which can be built.

By utilizing a larger tube diameter size in the upper walls of thefurnace, in accordance with the present invention, it is possible toobtain a lower mass flow rate for the circulating fluid in the upperwalls and a correspondingly lower velocity head. Thus, friction andshock losses in the upper portion of the furnace which are directlyproportional to the velocity head are respectively lower.

Accordingly, it is an object of the present invention to provide agenerator design which enables building a natural circulation generatorof greater capacity.

Another object of the present invention is to provide a naturalcirculation generator of greater capacity in which the likelihood offailure of tubes in the furnace wall circuitry is reduced.

Still another object of the present invention is to provide a naturalcirculation generator having a high circulation ratio in which thethroughput steam flow per foot of periphery in the furnace walls of thegenerator is increased.

A still further object of the present invention is to provide a largecapacity natural circulation generator in which conventional tubematerials can be employed.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described andparticularly pointed out in the claims, the following description andthe annexeddrawingsetting forth in detail a certain illustrativeembodiment of the invention, this being indicative, however, of but onlyone of the various ways in which the principles of the invention may beemployed.

In such annexed drawing:

FIG. 1 is a section, elevation view illustrating a natural circulationgenerator in accordance with the concepts of the invention;

FIG. 2 is an enlarged, perspective view of a portion of the furnaceenclosure tube wall of the generator of FIG. 1; and

FIG. 3 is an enlarged partial elevation view of a tube wall of thegenerator of FIG. 1 taken in zone B-B of the generator furnaceenclosure.

Referring to the drawing, the vapor generator in accordance with thepresent invention is broadly indicated with the letter A, and comprisesa vertically extending rectangular shaped radiant furnace area B havingan upper gas exit C, and a convection area D which leads downwardly fromthe gas exit. A burner zone E occupies the furnace area of the generatorimmediately above hopper F. The flow of hot gases is upwardly in thefurnace area from the burner zone, through the convection areas of thegenerator to the generator outlet G, and from there to a conventionalair heater H for heat exchange between hot gases and incoming air forthe burners.

The present invention is concerned primarily with the construction ofthe furnace portion B of the generator.

The vapor generator furnace comprises an upright, rectangular enclosure12 defined by front and rear walls l4, 16. Side walls 18 (only one ofwhich is shown) extend between the front and rear walls, the entireenclosure leading vertically from the bottom hopper F to an inclinedroof 22. Beneath the roof 22, the rear wall is bent inwardly andthenrearwardly to provide a reverse arch 24, above which the wall isbranched at the furnace exit C into parallel but spaced apart, openscreen tube panels 26, 28 which permit the flow of gas from the furnacearea of the generator to the generator convection area.

In the convection area, frequently referred to as the heat recoveryarea, the generator comprises a generally, rectangular enclosure 30separated into two gas passes 32, 34 by a division wall 36. The rearmostof the two gas passes houses the generator reheater section 38, theother of the gas passes containing superheater and economizer sections40, 42, respectively.

The upper zone 44 of the furnace, above the burner zone E, contains aplurality of J-shaped division wall panels 46 in the front of thefurnace area, and a pendant finishing superheating section 48 above thearch 24 in the rear wall of the generator. The pendant finishingsuperheater is positioned immediately in front of the screen panels 26,28 leading to the convection area of the generator.

The flow of the fluid being heated in the generator is from theeconomizer 42 into the lower inlet headers 50 for the radiantly heatedwalls 14, 16 and 18 of the generator enclosure. The flow in these wallsis upwardly in generally parallel tubes of the walls into risers 52 atthe top of the generator and from there into the steam and water drums54, 56. Liquid separated from the flow in the drums is recycled bydowncomers 58 to the lower inlet headers of the enclosure, by naturalcirculation.

Vapor separated from the flow in the steam and water drums istransmitted by conduits 60 into an inlet header62 for the downwardlyinclined roof 22 of the generator. At the end of the roof panel, aheader 64 divides the fluid for flow into the walls of the convectionenclosure 30 of the generator and into the division wall 36 of theconvection area. These walls terminated in a lower header 66 at thebottom of the convection area, from which the flow is transmitted intothe bank 40 of the primary superheating tubes. From there via additionalheaders and conduits, the flow is into the division wall panels 46, andpendant finishing superheating sections 48 in that order.

It is a feature of the invention that a membrane-type wall constructionillustrated in detail in FIG. 2 is employed substantially throughout allof the furnace walls of the generator, except for the screen tube panels26 and 28 at the gas exit C, and except where the walls are penetrated,forinstance, by the burners and division wall panels. This membrane-typewall construction is obtained by welding together a plurality of finnedtubes 68 along their lengths so that the enclosure is substantiallygas-tight.

In accordance with the present invention, the furnace enclosure isdivided into higher and lower temperature tube panel sections B and B",the higher temperature section B being roughly coextensive with theenclosure burner zone E, and extending from near the bottom of thefurnace to an elevation above the burners 70; the lower temperaturesections B" constituting the remainder of the furnace enclosure up tothe roof and being roughly coextensive with the upper zone 44 of theenclosure. As shown in FIG. 3, the tube diameters are increased in thepanel sections in a transition area 72 between the burner and lowertemperature zones, being smaller in diameter in the burner zone andlarger in diameter in the upper zone.

In a particular example, in a furnace about 41 feet by 91 feet in crosssection, 36 burners are positioned in the front wall of the generator atfour elevations. At an elevation about 20 feet above the uppermost rowof burners, just below the elevation of the division wall panels andarch of the furnace, the transition from smaller diameter tubes tolarger diameter tubes occurs. The smaller diameter tubes are 3 inches inoutside diameter on 3% inch centerlines, and the larger diameter tubesare 3% inches in outside diameter, also on 3% inch centerlines, so thatthe centerlines for the tubes are in alignment for the full elevation ofthe furnace. At the furnace gas exit C, the tubes of the rear wall aredivided into parallel spaced apart screen located on 7% inch centers,providing adequate space for the flow of gases from the furnace into theconvection area.

It is apparent that to obtain increased capacity, it is necessary toincrease the heat absorption surface in the furnace, requiring increasedfurnace height and periphery. It is contemplated that the unit will beoil fired, and oil firing has a relatively high heat release ratecompared to coal firing (although less than gas firing). This will cutdown on the furnace size somewhat, but not enough to attain thenecessary circulation ratio. In addition, it has the disadvantage thatthe high heat release rate can cause overheating of the tube surface inthe burner zone, requiring the use of expensive alloy tubing in thiszone.

Employing smaller diameter tubing in the high heat intensity burner zonehas the advantage that the relatively high flow in the tubes providesfor better cooling in this zone. In addition, the smaller diameterpermits the use of thinner walled tubing for better heat transfer andcooling of the tubes. As a result, plain carbon steel tubes can be usedin this zone.

The use of large diameter tubing in the upper zone has the advantage ofincreasing the surface area for heat absorption, for a given furnaceperiphery, and to thereby obtain a relatively high throughput steam flowper foot of periphery, in the order of 20,000 lbs. per hour-foot in theunit described. It also reduces friction and other losses to insure ahigh circulation ratio in the generator, sufficiently high so that theleast favorable circuit receives an adequate water supply.

The use of larger diameter thicker wall tubing in the upper zone of thefurnace still further has the advantage of offering better support forthe walls of the generator, permitting the generator to betop-supported.

Positioning the division wall and finishing superheater surfaces in theupper furnace has the advantage that it reduces gas temperature at thegas exit to within design limits. In this respect, it permits limitingthe amount of surface area in the furnace perimeter to that necessary toattain the required circulation ratio. Location of these surfaces in ahigh heat intensity zone also facilitates control of superheattemperatures.

It should be understood that while the present invention is not limitedto an all welded fin-tube construction, the use of different diametertubes in the furnace of the fin-tube vapor generators becomes feasible,since if a conventional skin casing were employed, the wide gap at thebottom of the furnace between the tubes would make it difficult to sealthe furnace.

As an example of the increased capacity attainable in a naturalcirculation generator by employing the concepts of the invention, theunit described above has a capacity of about 900 megawatts, as comparedto a maximum of about 600 megawatts for previous units.

A latitude of modification, change and substitution is intended in theforegoing disclosure and in some instances some features of theinvention will be employed with a corresponding use of other features.Accordingly, it is appropriate that the appended claims be construedbroadly and in a manner consistent with the spirit and scope of theinvention herein.

What is claimed is:

l. A natural circulation generator of increased capacity comprising arectangular furnace enclosure; a plurality of parallel upright tubeswelded together along 5 their lengths forming the walls of theenclosure;

burners in at least one of the walls of the enclosure near the bottom ofthe enclosure;

gas exit means at the top of the enclosure;

the tube sizes in the lower portion of the enclosure being of onediameter, those in the upper portion of the furnace, above the burners,being of a larger diameter to provide an increased throughput steam flowper foot of furnace periphery, whereby the friction and shock losses arereduced in said upper portion and a higher circulation ratio is achievedin the entire circuit.

2. The generator of claim 1 wherein said burners are in the front wallof the furnace, the gas exit means being in the rear wall of the furnacefurther including division wall means at least in the upper portion ofthe furnace; and pendant superheater means in the upper portion of thefurnace in front of said gas exit means. 3. The generator of claim 2wherein said division wall means is steam cooled.

4. The generator of claim 2 wherein said division wall means is in theflow circuit of the generator immediately upstream of the pendantsuperheater means.

5. The generator of claim 4 including a convection zone connected withthe furnace enclosure gas exit means, the convection zone housingreheater, superheater and economizer tube surface.

6. The generator of claim 2 wherein the furnace enclosure comprisesfront, rear and side walls, the tubes of the rear wall immediately belowthe gas exit means being bent inwardly to the furnace and thenrearwardly to form a reverse arch penetrating into the furnace fordeflection of hot gases across said division wall means.

7. The generator of claim 6 wherein said division wall means comprises.l-shaped panels of tubes having ends penetrating the front wall of thegenerator and the roof of the generator, the elevation of penetration ofthe front wall being above the area of transition in the enclosure walltubes from the smaller diameter tubes to the larger diameter tubes.

8. The generator of claim 2 wherein said larger diameter and smallerdiameter enclosure wall tubes are on vertically aligned centers.

9. The generator of claim 8 wherein said enclosure wall tubes are weldedalong their lengths to provide a gas-tight construction.

10. A natural circulation generator of increased capacity comprising arectangular furnace enclosure;

55 a plurality of parallel, upright, finned tubes welded together alongtheir lengths forming the walls of the enclosure;

burners in at least one of the walls of the enclosure near the bottom ofthe enclosure;

gas exit means at the top of the enclosure;

the tube sizes in the lower portion of the enclosure being of onediameter, those in the upper portion of the enclosure being of a largerdiameter;

the tube diameter in the lower portion of the enclosure beingsufficiently small to permit the use of plain carbon steel tubing;

the tube diameter in the upper portion of the enclosure beingsufficiently larger in diameter to obtain that circulation ratio andthroughput steam flow per hour-foot of furnace periphery required;

further including division wall means at least in the upper portion ofthe furnace; and

pendant superheater means in the upper portion of the furnace in frontof said gas exit means.

11. The generator of claim 10, the burner means being for 75 oil or gasfiring.

1. A natural circulation generator of increased capacity comprising arectangular furnace enclosure; a plurality of parallel upright tubeswelded together along their lengths forming the walls of the enclosure;burners in at least one of the walls of the enclosure near the bottom ofthe enclosure; gas exit means at the top of the enclosure; the tubesizes in the lower portion of the enclosure being of one diameter, thosein the upper portion of the furnace, above the burners, being of alarger diameter to provide an increased throughput steam flow per footof furnace periphery, whereby the friction and shock losses are reducedin said upper portion and a higher circulation ratio is achieved in theentire circuit.
 2. The generator of claim 1 wherein said burners are inthe front wall of the furnace, the gas exit means being in the rear wallof the furnace further including division wall means at least in theupper portion of the furnace; and pendant superheater means in the upperportion of the furnace in front of said gas exit means.
 3. The generatorof claim 2 wherein said division wall means is steam cooled.
 4. Thegenerator of claim 2 wherein said division wall means is in the flowcircuit of the generator immediately upstream of the pendant superheatermeans.
 5. The generator of claim 4 including a convection zone connectedwith the furnace enclosure gas exit means, the convection zone housingreheater, superheater and economizer tube surface.
 6. The generator ofclaim 2 wherein the furnace enclosure comprises front, rear and sidewalls, the tubes of the rear wall immediately below the gas exit meansbeing bent inwardly to the furnace and then rearwardly to form a reversearch penetrating into the furnace for deflection of hot gases acrosssaid division wall means.
 7. The generator of claim 6 wherein saiddivision wall means comprises J-shaped panels of tubes having endspEnetrating the front wall of the generator and the roof of thegenerator, the elevation of penetration of the front wall being abovethe area of transition in the enclosure wall tubes from the smallerdiameter tubes to the larger diameter tubes.
 8. The generator of claim 2wherein said larger diameter and smaller diameter enclosure wall tubesare on vertically aligned centers.
 9. The generator of claim 8 whereinsaid enclosure wall tubes are welded along their lengths to provide agas-tight construction.
 10. A natural circulation generator of increasedcapacity comprising a rectangular furnace enclosure; a plurality ofparallel, upright, finned tubes welded together along their lengthsforming the walls of the enclosure; burners in at least one of the wallsof the enclosure near the bottom of the enclosure; gas exit means at thetop of the enclosure; the tube sizes in the lower portion of theenclosure being of one diameter, those in the upper portion of theenclosure being of a larger diameter; the tube diameter in the lowerportion of the enclosure being sufficiently small to permit the use ofplain carbon steel tubing; the tube diameter in the upper portion of theenclosure being sufficiently larger in diameter to obtain thatcirculation ratio and throughput steam flow per hour-foot of furnaceperiphery required; further including division wall means at least inthe upper portion of the furnace; and pendant superheater means in theupper portion of the furnace in front of said gas exit means.
 11. Thegenerator of claim 10, the burner means being for oil or gas firing.